Fusel oils are formed as a by-product of alcoholic fermentation, and consist of a mixture of several alcohols comprised mainly of amyl alcohols along with lesser amounts of propanol, n-butanol, and isobutanol depending upon the purification process employed. In some cases, these fusel oils are re-added back into an ethanol product at some level, burned as fuel, or further purified and sold on the open market. Depending on the carbohydrate source for the fermentation process, and the organism used, fusel oil levels are typically between 0.2-3.0% as a relative percent of the target alcohol produced. Generally, fusel oil alcohols are considered waste compounds in the production of bio-ethanol, and are often burnt as fuel for heating distillation columns.
Some metabolites involved in fermentation processes have been identified as a source of higher alcohols such as fusel oils. Ehrlich (1907) and Klosowski et al. (2010) have attributed the production of higher alcohols to amine nitrogen assimilation. Considering the high content of amyl alcohols in fusel oil (Maiorella et al., 1981), it is important to mention the role played by leucine and isoleucine as source molecules of 3-methyl-1-butanol and 2-methyl-1-butanol, respectively (Ribereau-Gayon et al., 2006). Roehr (2001) indicates that fusel oil is formed from α-keto acids, derived from amino acids. Pfenninger (1963) and Roehr (2001) compared fusel oil composition based on the feedstock for fermentation. Their results indicate that butanol and i-amyl alcohols content increases when molasses and fruits are the raw material for fermentation. However, those authors point out the importance of pH in the fermenter in the production of higher alcohols.
Raw fusel oil is a relatively viscous liquid with a dark-reddish color, and a very unpleasant odor. As a result of these properties, the direct utilization of fusel oil as a solvent has been very limited. In some countries it is burned to supply energy for the ethanol processing plants. In Turkey, it is used mainly for denaturation of alcohol, or for removing the foam from molasses during sugar manufacturing. A substantial portion of it, however, has generally been discarded. Recent studies have suggested that several alternative uses for fusel oils are possible. For example, acetic acid and butyric acid esters, major alcohol components of fusel oil, have economic value as chemicals for flavor and fragrance manufacturing (Ceylan et al, 1997). Especially ethyl butyrate is in high demand as a component of pineapple-banana flavors in the food industry. Seino et. al (1984) investigated the enzymatic synthesis of the carbohydrate esters of fatty acids. Gilles, et. al. (1987) and Welsh and Williams (1989), studied enzymatic esterification of fusel oils with acetic acid and butyric acid. Generally, lipase enzyme was used in these studies. Ghuiba et. al. (1985) investigated chemical esterification of fusel oil with polybasic acids with high molecular weights at relatively high temperatures. They reported the esters synthesized under these conditions are very compatible with polyvinyl chloride as plasticizers. Ay et. al. (1994) reported that fusel oil can be used for the purification of phosphoric acid produced by the wet method. They also reported that especially iso-amyl alcohol and iso-butyl alcohol are selective for extraction of the acid. Hasan et. al. (1993) reported that the indigenously prepared solvent tri-isoamyl phosphate (TAP) obtained from fusel oil has been successfully utilized to extract titanium (IV) from its aqueous solutions. Pereira et al (2013) reported the conversion of main fusel oil components (3-methyl-1-butanol, 2-methyl-1-butanol, and isobutanol) to organic alkyl carbonates by carbon dioxide fixation.
Fusel oil can be also used as a raw material for production of amyl and butyl alcohols which have some different but significant applications: The work of Ogonowski and Sikora (2000) deals with catalytic conversion of methanol and i-butanol into ethers. Similarly, Klier et al. (1997) studied the conversion of higher alcohols into different ethers over multifunctional catalysts. Vaze et al. (1997) presented an alternative of valorization consisting on the electrochemical oxidation of alcohols to produce carboxylic acids. Mitra et al. (1997) studied the alkylation of aromatic compounds with alcohols such as butanol and i-butanol in order to provide green feedstocks to the pharmaceutical industry. Garcia et al. (1997) and Liaw et al. (1998) also offer alternatives for using butanol and i-amyl alcohol in pharmaceutical applications.
Moreover, in the last decade, fusel oils have been used for the manufacture of bio-based products with the advantage of being environmentally safe, renewable, and in some cases biodegradable. Özgülsün et al. (2000) studied the esterification reaction of oleic acid with a fraction of fusel oil from molasses to produce lubricating oil. Dörmo et al. (2004) synthesized a bio-lubricant from fusel oil by enzymatic esterification. Güvenç et al. (2007) and Bandres et al. (2010) performed the syntheses of bio-based solvents to obtain acetates, carbonates and i-valerates with i-amyl alcohol from fusel oil.